1. Field of the Invention
This invention relates to the field of charged particle optics, and in particular to electron optical structures used for wafer inspection.
2. Description of the Related Art
Defect inspection of semiconductor wafers and masks for IC manufacturing is an accepted production process for yield enhancement. The information obtained from a wafer defect inspection tool can be used to flag defective wafers for reprocessing, or to correct wafer processing parameters. The systems used for inspection typically use light optics and such systems are limited in resolution due to the wavelength of the measuring light (200-400 nm). As the feature sizes on semiconductor wafers drop below 0.18 μm, the resolution requirements of the defect inspection tools become more demanding.
To overcome this resolution limit, electron beam inspection systems have been designed. Electron beam systems can have much higher resolution than optical systems because the wavelength of the electrons can be in the Ångstrom regime. Electron beam systems are limited in the speed at which they can inspect wafers—in present systems, throughputs of approximately 30 minutes per square cm have been reported (Hendricks et al, SPIE vol. 2439, pg. 174). Thus to inspect an entire 300 mm diameter silicon wafer, approximately 70 hours would be required. These systems can be used in a sampling mode where only a small percent of the wafer area is inspected, thereby increasing throughput substantially. These systems have been effective for research and product development, but are impractical for use in-line in a semiconductor fabrication facility.
Multi-beam electron beam inspection tools have been proposed as viable in-line inspection tools, where high throughputs are required. These tools have electron optical designs that vary from multiple electron beam columns built on a single semiconductor wafer to a number of physically separate electron beam columns clustered together. Multiple columns built on a single wafer must be “perfect” columns as-fabricated, since there is no facility for adjusting the physical electron optical alignment of any column; further, the electron optical components of the column are restricted to what can be fabricated on a semiconductor wafer. The cluster of physically separate electron beam columns is costly to manufacture. Clearly, there is a need for an electron optical design that provides more flexibility in the choice of electron optical components than the single wafer design, combined with the ability to independently align electron optical components for each column, and there is a need for a design with a lower manufacturing cost than the cluster of physically separate columns.